robert de niro in awakenings (not really retarded but semi catatonic looks same in a movie)

dustin hoffman in rainman

sean penn in i am sam

adrien brody in the village

john malkovich in of mice and men

leonardo di caprio in what’s eating gilbert grape

billy bob thornton in sling blade

john matuszak in the goonies

christopher lambert in greystoke: the legend of tarzan, lord of the apes (not actually retarded but effect on screen is quite similar)

ben stiller in tropic thunder (simple jack)

to be really good actor you have to play a character with a retardation.